"They've taken a very good player with a huge future. I'm convinced he'll succeed at Real Madrid."

But while harbouring no doubts over Militao's ability to play at the highest level, Casillas warned that Madrid fans may have to show the youngster patience as he adapts to a new league.

"Porto gambled on him when he was very young and he showed a big personality," Casillas added.

"He's very noble and wants to do things the right way, I'm just a little afraid for his adaptation because in Spain it's different.

"I always tell him that when he arrives in Madrid, he'll realise what it means."

Brazil centre-back Militao has made 34 appearances for Porto across all competitions this season, and will have another chance to impress when the Primeira Liga giants take on Liverpool in the Champions League quarter-finals in April.